In 2009, Amazon had 2 million Prime members. In 2011, Prime has 5 million members out of a total 121 million Amazon customers. Although that represents only four percent of all customers...

Amazon Prime members pay a yearly fee (moms & students can receive up to a year for free) to get complimentary two-day shipping on just about anything they buy from Amazon. Amazon doesn't disclose details of the Prime program publicly but according to estimates Amazon Prime members:

Increase their purchases on Amazon from $400 a year to $900 a year after they join. (source)

The Prime promotion grew Amazon’s shipping costs 55% to $4 billion last year, which is well over the $1.55 billion Amazon gets in shipping fees from customers. (source)

Amazon spends more than $90 a year for each Prime customer, losing $11 annually for each subscriber. Of the $90, $55 comes from shipping costs and $35 comes from acquiring digital video content. (source)